fix: replace source <(gstack-slug) with eval for bash compatibility

Under bash with set -euo pipefail, source <(cmd) process substitution
doesn't reliably set variables in the caller's scope. The variables
stay empty and -u (nounset) crashes the script. eval "$(cmd)" works
correctly in both bash and zsh.

Fixes: gstack-review-read, gstack-review-log, gstack-slug comment,
gen-skill-docs.ts resolver functions, and regression tests.

bin/gstack-review-log

@@ -3,7 +3,7 @@
 # Usage: gstack-review-log '{"skill":"...","timestamp":"...","status":"..."}'
 set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-source <("$SCRIPT_DIR/gstack-slug" 2>/dev/null)
+eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
 mkdir -p "$GSTACK_HOME/projects/$SLUG"
 echo "$1" >> "$GSTACK_HOME/projects/$SLUG/$BRANCH-reviews.jsonl"

bin/gstack-review-read

@@ -3,7 +3,7 @@
 # Usage: gstack-review-read
 set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-source <("$SCRIPT_DIR/gstack-slug" 2>/dev/null)
+eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
 cat "$GSTACK_HOME/projects/$SLUG/$BRANCH-reviews.jsonl" 2>/dev/null || echo "NO_REVIEWS"
 echo "---CONFIG---"

bin/gstack-slug

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # gstack-slug — output project slug and sanitized branch name
-# Usage: source <(gstack-slug)  → sets SLUG and BRANCH variables
+# Usage: eval "$(gstack-slug)"  → sets SLUG and BRANCH variables
 # Or:    gstack-slug            → prints SLUG=... and BRANCH=... lines
 #
 # Security: output is sanitized to [a-zA-Z0-9._-] only, preventing